The online access thing is a bit of a pain. Played a bit the first few days... and it worked but the "Achievement Service" went offline, so eventually I had to redo the first few missions on Brutal just so it would give me credit for having done it, even though the game itself listed my completion time for that difficulty on every level.
I wouldn't say low tier units become useless. You can pretty much march through the game with Zerglings and Roaches. I did mix in some Ultralisks and Aberrations near the end.